1. Field of the Invention
The invention relates to a fuel cell system, and more particularly to a fuel cell system including a fuel cell having improved durability.
2. Description of the Related Art
In fuel cells, electric energy is generated by the electrochemical reaction that occurs in a membrane-electrode assembly (hereinafter, referred to as “MEA”). The MEA includes an electrolyte and electrodes (a cathode and an anode). One of the electrodes is provided on one side of the electrolyte, and the other of the electrodes is provided on the other side of the electrolyte. The electric energy generated is taken out of the MEA via separators. A separator is provided on one side of the MEA, and another separator is provided on the other side of the MEA. A polymer electrolyte fuel cell (hereinafter, referred to as “PEFC”), which is one type of fuel cell, is used in a co-generation system for home use, an automobile, and the like. The PEFC can operate at low temperatures. Generally, the PEFC operates between temperatures of 50 to 100° C. The PEFC has a high energy-conversion efficiency of 50 to 60%. It takes only a short time to start the operation of the PEFC. Also, PEFC systems are compact and light. Therefore, the PEFC is suitable for use as a power source for an electric vehicle, a portable power source, and the like.
An individual PEFC cell includes an electrolyte membrane, a cathode, an anode, and separators. Each of the cathode and the anode includes a catalyst layer and a diffusion layer. A single PEFC cell has a theoretical electromotive force of 1.23 volts. Because an individual cell has such a low electromotive force, it cannot supply sufficient power to move an electric vehicle and the like. Therefore, ordinarily, a fuel cell stack is used. A fuel cell stack includes a stack body, end plates, and the like. The stack body is formed by stacking individual cells in series. The end plates are disposed at both ends of the stack body in the direction in which the individual cells are stacked.
It has been found that when the fuel cell generates electric power, hydrogen peroxide is generated in the catalyst layer in the unit cell, which causes the polymer electrolyte of the MEA to deteriorate. The deterioration of the polymer electrolyte may reduce the electric power generation performance of the fuel cell. Therefore, it is desirable to suppress the deterioration of the polymer electrolyte by reducing the amount of hydrogen peroxide in the MEA, thereby improving the durability and the electric poser generation performance of the fuel cell.
Japanese Patent Application Publication No. JP 2003-100308 A (hereinafter “JP 2003-100308 A”) discloses an example of a technology for improving the electric power generation performance of a fuel cell. More specifically, JP 2003-100308 A discloses a cathode catalyst for a fuel cell. In the cathode catalyst, a carrier that supports Pt further supports CeO2. By using this catalyst, the reduction reaction of oxygen proceeds at an increased rate. Also, Japanese Patent Application Publication No. JP 2003-86192 A (hereinafter “JP 2003-86192 A”) discloses a fuel cell in which a cathode electrode includes a catalyst/oxide/polymer electrolyte thin layer. The catalyst/oxide/polymer electrolyte thin layer is made of an oxide, a catalyst, and a solid polymer electrolyte material. In this fuel cell, cross over is suppressed. Thus, the reduction of the electric power generation performance due to the cross over can be prevented.
Various substances, such as reaction gas supplied from the outside of the fuel cell and water generated inside the fuel cell, move in the MEA. Therefore, even when a catalyst containing CeO2 is mixed into the catalyst layer in advance according to the technology disclosed in JP 2003-100308 A, the catalyst containing CeO2 is likely to be discharged from the MEA by the various materials. Thus, even when employing the technology disclosed in JP 2003-100308 A, it is difficult to maintain the effect of CeO2 in the MEA, and to improve the durability of the fuel cell. Also, even when employing the technology disclosed in JP 2003-86192 A, it is difficult to improve the durability of the fuel cell.